Switch "apt-cacher-ng" to "debian:stretch-slim" for a smaller base and update tini to 0.13.0
Switch "apt-cacher-ng" to "debian:stretch-slim" for a smaller base and update tini to 0.13.0

jakirkham/dockerfiles,jakirkham/dockerfiles,jakirkham/dockerfiles,jakirkham/dockerfiles,jakirkham/dockerfiles